Oh, yeah, the base of the drop-inlet is in the dam, of course. I wouldn't underestimate the power of freeze-thaw-freeze cycles even on concrete. And that water is moving through part of the dam to get to your pipe, so you can bet there will be some erosion there.
I'm pretty sure I'd be draining down that water level so I could get that thing patched for good. And hydraulic cement is good stuff - won't shrink when it dries. I apologize I don't have time to do it right now, but if you do some searching on this site you'll find discussions on using trash pumps or temporary siphon systems to lower water levels.
